Fly ash is the by-product of burning pulverized coal. The chemical content and particle size of fly ashes vary widely in accordance with the source of the coal, the fineness to which it is ground, and the furnace within which it is burned.
The chemical composition of fly ash may vary generally, as follows:
______________________________________ Class F Class C Component Percent by Weight Percent by Weight ______________________________________ SiO.sub.2 35-55 20-40 Al.sub.2 O.sub.3 15-35 5-15 FeO 3-25 1-10 Fe.sub.2 O.sub.3 CaO 0.5-8 0-35 MgO 0.5-3 0-5 TiO.sub.2 1-3 1-3 Na.sub.2 O 0-1.5 0-1.5 K.sub.2 O 0-3 0-3 SO.sub.3 1-3 0-6 C 0.5-20 0.5-20 H.sub.2 O 0.5-0.7 0.5-3 ______________________________________
Fly ash can be used for a variety of purposes. Principally, it is used as a replacement for cement in Portland Cement concrete. Additionally, for example, it has been used as filler or extender for plastics and asphalt, as a source of carbon for waste water purification, and as a magnetite source for coal cleaning.
Many class F fly ashes collected throughout the United States have carbon content (measured as Loss on Ignition--LOI) of about 0.5% to 4.0%. These ashes may be sold as an acceptable mineral admixture for use in Portland cement concrete under the standards set forth in ASTM C-618-92a which limits maximum LOI for classes F and C fly ash at 6.0%. As such, these ashes are a commercially attractive commodity, with the sale of same by electrical utilities and industrial concerns providing lucrative revenue.
Unfortunately, not all fly ashes possess such low carbon content as to pass the standards provided in ASTM C-618. These non-conforming fly ashes have LOI content of &gt;6%, sometimes on the order of &gt;6%-20%.
In addition to lack of conformity with the requirements of ASTM C-618, high carbon content fly ash has been shown to cause undesirable reduction in entrained air in concrete and it provides an oily or dark surface appearance on finished concrete surfaces since free carbon floats to the surface during finishing. Moreover, high carbon content fly ash exhibits reduction in desirable pozzolanic reactivity. Use of fly ash that is not in conformity with the aforementioned ASTM standard as a mineral admixture in Portland cement concrete and its consequent failure to meet material specifications raise serious liability problems.
Due to the limitations upon its ultimate end use, such high carbon fly ash also poses a disposal problem.
In light of the above, there exists a need in the industry to provide an inexpensive method and apparatus by which the carbon content (as measured by LOI) of high carbon containing fly ash mixtures can be reduced so that the resulting fly ash will conform with or exceed the above ASTM standard; therefore providing that the fly ash can be acceptably sold for and used as a mineral admixture for Portland Cement concrete products and other purposes.
Fly ash, collected as a by-product of burning pulverized coal which also has an unacceptable LOI content, presents a peculiar problem which inhibits separating a substantial portion of the carbon particle content from the alumina and silicate particle contents. This problem of separation includes the fact that fly ash is hygroscopic. Moisture, even in very small amounts, creates a surface bond between fly ash particles, including bonding of the non-carbon particles to carbon particles and the entrapment of carbon particles with co-bonded silicate and/or alumina particles. Thus, moisture contents as low as 0.9%, causes particle bonding, and can result in fly ash forming in bulk, a weight supporting surface due to the cohesion or aggregation of the individual particles.
When fly ash is formed, due to the extreme heat of combustion, it may be said to be totally dry or devoid of measurable moisture content. It is collected, conventionally, in a baghouse or by an electrostatic precipitator while it is yet hot, and is pneumatically conveyed to closed storage bins. However, due to its hygroscopic nature, in storage and in subsequent transport, the fly ash can readily pick up moisture from the surrounding air or from the air used for conveying the fly ash, which moisture will severely impede and hinder the separation of the carbon particles from the remaining particles, primarily silica and alumina silicate particles.
A further characteristic which impedes the separation of carbon particles from the remainder of electrostatic fly ash content resides in the fact that the carbon particles are formed and distributed throughout a wide range of particle sizes, including relatively easily separated large carbon lumps in the order of 100.mu. or more, together with a large quantity of exceedingly fine particles, as small as the finest of the fly ash particles, typically less than 10.mu.. The latter are much more difficult to separate and collect, and can comprise a substantial portion of the actual carbon content by weight and volume.
It is known that fly ash may be subjected to electrostatic separation processes, and that carbon particles can be separated from the bulk of the fly ash due primarily to the fact that the carbon particles are relatively more conduction than the bulk of the alumina and silicate particles. However, prior separation techniques and apparatus have failed to recognize or address the special characteristics of fly ash which impede or present effective separation.